The present invention relates to a method and apparatus for detecting anomalies in earth resistivity, and particularly to such method and apparatus for detecting tunnels or other void spaces within the earth.
Systems for detecting tunnels within the earth have previously been suggested which require the measurement of mutual impedance between a long current-carrying wire placed at the earth's surface and the voltage in a receiving loop located away from the wire. The applicability of such systems, however, is generally limted to tunnels that are not too deep, have fairly large dimensions, and contain linear conductive structures, such as steel tracks (e.g., C. Bufe, et al, "Geophysical Studies in Tunnel Location" ECOM TR-00013-171, prepared by Willow Run Lab., Univ. of Michigan, September 1969; G. V. Keller, F. C. Frieschknecht "Electrical Methods in Geophysical Prospecting" Pergamon Press, New York 1966). There are increasing military as well as commercial requirements for a system enabling detection of tunnels which are deeper and smaller than those which may be practically detected by mutual impedance systems and which may not be provided with steel tracks or other linear conductive structure. In addition, it would be advantageous to realize a tunnel detection system which is free from errors which derive from errors in wire current and loop voltage.